T cell co-receptor signaling is an important mechanism for tightly regulating immune responses. The cell surface molecules for co-signaling (including co-stimulation and co-suppression) can be divided into two main families: immunoglobulin (Ig) superfamily and tumor necrosis factor (TNF)-tumor necrosis factor receptor (TNFR) superfamily. Generally, the activation of T cells is dependent on the antigen peptides presented by HLA class I or II molecules. Co-receptor signaling will either increase or prevent this activation. For example, the initiation and maturity of the lymphocytes in peripheral lymphoid organs can be promoted, or their response effects in an organism can be enhanced by such as the activation of CD28 or 4-1BB with agonists through the unique co-stimulation pathways. Immune activation could also be achieved by blocking co-suppression signaling pathways with antagonists, such as programmed cell death protein 1 (PD-1), B7 homolog 1 (B7-H1, also referred as PDL1) pathway, cytotoxic T lymphocyte antigen 4 (CTLA4), B/T lymphocyte attenuator (BTLA) and other pathways. These co-suppression signaling pathways play important roles in the regulation of immune tolerance, which provide negative signals that limit, terminate and/or attenuate immune responses. Immune activation mediated by co-stimulation receptors is activated by stimulating membrane-proximal kinase and produces phosphorylation cascade amplification, while co-suppression receptors such as CTLA4, PD1 and B/T lymphocyte attenuator (BTLA) recruit phosphatase to reverse the phosphorylation which is induced by immune activation. Immunomodulatory biological formulations can be widely used in the treatment of immune-related diseases, to inhibit the immune hyperfunction caused by transplant rejection, autoimmune disease or inflammatory disease, or to stimulate the immune response to immune hypofunction such as cancer, chronic bacterial infection and virus infection etc. Unlike the mainstream therapies based on monoclonal antibodies and recombinant fusion proteins, i.e. by neutralization or consumption of target antigens or target-positive cells, the immunomodulatory biological formulations regulate the signals for the antigen-specific T cell receptor (TCR) and B cell receptor(BCR) mainly by binding and regulating the signal molecules on the surface of the host's immune cells, so as to control the direction and intensity of the lymphocyte response.
PD-1 gene is up-regulated when T cell hybridoma undergoes apoptosis, and is named as programmed cell death protein 1. PD-1 (CD279) is expressed in activated T cells, B cells, and activated myeloid cells (Ishida Y, Agata Y, Shibahara K, Honjo T. EMBO J. 1992, 1: 3887-3895), and is also expressed in activated macrophages, DC and monocytes, but is not present in their immature cells (Agata Y, et al. Int. Immunol. 1996, 8: 765-772; Said E A, et al. Nature Med. 2010, 16: 452-459). These up-regulated expressions of PD-1 on cell surface can inhibit acquired immune and innate immune responses of an organism. PD-1 intracellular domain contains two tyrosine sites, one is immunoreceptor tyrosine-based inhibitory motif (ITIM), and the other is immunoreceptor tyrosine-based switch motif (ITSM). The phosphorylation of the tyrosine on ITSM recruits tyrosine phosphatase SHP2 and/or SHP1. These phosphatases will dephosphorylate ZAP70, CD3 and PKC, thereby attenuating T cell signals. PD-1 mainly inhibits T cell and B cell proliferation by causing the cells to be arrested in GO/G1 phase, and inhibits the cytokine production in T cells. The animals of PD-1 expression deletion develop various antoimmune phenotypes, including autoimmune cardiomyopathy, lupus-like syndromes with arthritis and nephritis (Nishimura et al. Immunity. 1999, H: 141-51; Nishimura et al. Science. 2001, 291: 319-22). In addition, PD-1 also plays an important role in autoimmune encephalomyelitis, systemic lupus erythematosus (SLE), graft-versus-host disease (GVHD), diabetes type I and rheumatoid arthritis, etc. (Salama et al. J Exp Med. 2003, 198: 71-78; Prokunina and Alarcon-Riquelme, Hum Mol Genet. 1992, 13: R143; Nielsen et al. Lupus. 2004, 11: 510).
Two PD-1 ligands have been currently reported, PD-L1/B7H1/CD274 and PD-L2/B7-DC/CD273 (Freeman G J, et al. J. Exp. Med. 2000, 192: 1027-1034; Latchman Y, et al. Nature Immunol. 2001, 2: 261-268). PD-L1 is expressed at a low level in immune cells, such as B cells, dendritic cells, macrophages and T cells, and is up-regulated upon cell activation. PD-L1 is also expressed in non-lymphoid organs such as vascular endothelial cells, heart, lung, pancreas, muscle, keratinocytes and placenta etc. The expression of PD-L1 in non-lymphoid tissue reveals that PD-L1 may regulate the function of self-reactive T cells, B cells and myeloid cells in peripheral tissues, and may also participate in inflammatory responses of a target organ. The expression of PD-L1 is mainly regulated by interferon 1 or 2, which are also the major regulators of PD-L1 level in vascular endothelial cells and epithelial cells. PD-L1 is also expressed in tumor cells, and is closely associated to poor prognosis. Various viral infections can induce the expression of PD-L1 in host tissues in a high level. Although PD-L2 transcript is found in non-hematopoietic tissues such as heart, liver and pancreas, the expression of PD-L2/B7-DC on cell surface is only restricted to macrophages and dendritic cells, and depends on the production of IFNγ and Th2 cytokines. The expressions of PD-L1 and PD-L2 are also affected by different stimulations. PD-L1 on macrophages is induced by INFy, while PD-L2 is regulated by IL-4. A similar phenomenon also appears on dendritic cells. The study reveals that PD-L1 might preferentially regulate Th1 response, while PD-L2 would regulate Th2 cell response. PD-L1 and PD-L2 both can inhibit T cell proliferation, cytokine production and the adhesion mediated by β1/β2 integrin. PD-L2 can also trigger the reverse signaling of dendritic cells, thereby resulting in IL-12 production and T cell activation.
PD-L1-PD-1 regulation axis plays a key role in the control of human T cell activation and the maintenance of organism immune tolerance, and is also utilized by tumor cell as well as virus in chronic virus infection (Yao S, Chen L. Trends Mol. Med. 2006, 12: 244-246; Zou W, Chen L. Nature Rev. Immunol. 2008, 8: 467-477). PD-L1 is highly expressed in a variety of human cancer tissues (Dong et al, Nat. Med. 2002, 8: 787-9). The expression of PD-L1 is associated to the progression and poor prognosis of certain types of malignancies (Thompson R H, et al. Cancer Res. 2006, 66: 3381-3385). PD-L1-PD1 pathway has also been confirmed to promote T cell depletion (Zajac A J, et al. J. Exp. Med. 1998, 188: 2205-2213). PD-L1-PD1 pathway caused by tumors or viruses can achieve the avoidance of host immunological surveillance through a variety of mechanisms, including promoting T cell inactivation, fatigue, unresponsiveness and apoptosis, inducing Treg cell amplification, and enhancing intrinsic ability of tumor to resist killing and apoptosis. The interaction of PD-1 and PD-L1 mediated by cancer cells results in the reduction of tumor infiltrating lymphocytes, the inhibition of T cell proliferation mediated by T cell receptors, and increased immune escape (Dong et al. J. Mol. Med. 2003, 81: 281-7; Blank et al. Cancer Immunol. Immunother. 2005, 54: 307-314; Konishi et al. Clin. Cancer Res. 2004, 10: 5094-100).
To date, there is still no a satisfactory method which can induce effective immune response for a cancer patient to specifically block PD-L1-PD-1 regulation axis, and to provide activation of anti-tumor and anti-virus immune response. Therefore, there is an urgent need for development and design of a therapeutic method to specifically block PD-L1-PD-1 regulation axis, to overcome the immunosuppression of the patients having a cancer or a chronic infection.